On 7/18/19 7:17 PM, Yang Shi wrote:
> When both MPOL_MF_MOVE* and MPOL_MF_STRICT was specified, mbind() should
> try best to migrate misplaced pages, if some of the pages could not be
> migrated, then return -EIO.
>
> There are three different sub-cases:
> 1. vma is not migratable
> 2. vma is migratable, but there are unmovable pages
> 3. vma is migratable, pages are movable, but migrate_pages() fails
>
> If #1 happens, kernel would just abort immediately, then return -EIO,
> after the commit a7f40cfe3b7ada57af9b62fd28430eeb4a7cfcb7 ("mm:
> mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified").
>
> If #3 happens, kernel would set policy and migrate pages with best-effort,
> but won't rollback the migrated pages and reset the policy back.
>
> Before that commit, they behaves in the same way. It'd better to keep
> their behavior consistent. But, rolling back the migrated pages and
> resetting the policy back sounds not feasible, so just make #1 behave as
> same as #3.
>
> Userspace will know that not everything was successfully migrated (via
> -EIO), and can take whatever steps it deems necessary - attempt rollback,
> determine which exact page(s) are violating the policy, etc.
>
> Make queue_pages_range() return 1 to indicate there are unmovable pages
> or vma is not migratable.
>
> The #2 is not handled correctly in the current kernel, the following
> patch will fix it.
>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Some nits below (I guess Andrew can incorporate them, no need to resend)
...
> @@ -488,15 +496,15 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned
> long addr,
> struct queue_pages *qp = walk->private;
> unsigned long flags = qp->flags;
> int ret;
> + bool has_unmovable = false;
> pte_t *pte;
> spinlock_t *ptl;
>
> ptl = pmd_trans_huge_lock(pmd, vma);
> if (ptl) {
> ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
> - if (ret > 0)
> - return 0;
> - else if (ret < 0)
> + /* THP was split, fall through to pte walk */
> + if (ret != 2)
> return ret;
The comment should better go here after the if, as that's where fall through
happens.
> }
>
> @@ -519,14 +527,21 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned
> long addr,
> if (!queue_pages_required(page, qp))
> continue;
> if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> - if (!vma_migratable(vma))
> + /* MPOL_MF_STRICT must be specified if we get here */
> + if (!vma_migratable(vma)) {
> + has_unmovable |= true;
'|=' is weird, just use '='
> break;
> + }
> migrate_page_add(page, qp->pagelist, flags);
> } else
> break;
> }
> pte_unmap_unlock(pte - 1, ptl);
> cond_resched();
> +
> + if (has_unmovable)
> + return 1;
> +
> return addr != end ? -EIO : 0;
> }
>
...
> @@ -1259,11 +1286,12 @@ static long do_mbind(unsigned long start, unsigned
> long len,
> putback_movable_pages(&pagelist);
> }
>
> - if (nr_failed && (flags & MPOL_MF_STRICT))
> + if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
> err = -EIO;
> } else
> putback_movable_pages(&pagelist);
>
> +up_out:
> up_write(&mm->mmap_sem);
> mpol_out:
The new label made the wrong identation of this one stand out, so I'd just fix
it up while here.
Thanks!
> mpol_put(new);
>
